The next stage in rebuilding the destroyer fleet was replacing the remaining 36 old "second class" destroyers. Built between 1920 and 1926, these were worn out and obsolete. It was hoped that these could be replaced by 20 new destroyers built to a modified Tenryu design. This was officially a simplified version of the Tenryu but the description was only mildly acquainted with reality. The "simplification" consisted of deleting the catapult and seaplane storage (already done on all the existing Tenryu class), removing the aft mast and crane and reducing the number of high-angle directors from four to three but shifting one of those directors from a wing to a centerline position.
These largely cosmetic changes disguised a much more significant set of changes. The foremast was replaced by a large lattice mast that carried a long-range air search radar and electronic warfare equipment a wide-band signals receiver and two jammers. Below decks, the ships were equipped with a modern combat information center. Close observers noted that the bridge on these ships had been moved slightly aft and assumed this was to correct a trim problem but in reality, the design changes made room for the new command and control facilities.
All twenty Mutsuki class ships projected were ordered but only 16 were completed. The remaining four were re-ordered as the Kawari class.